The analysis of diffracted light coming from surface defects is a known technique at present in use for detecting and identifying surface defects (cracks, hairline cracks, flaws, cuts, scratches and the like) in mechanical parts. This analysis is normally carried out by illuminating the surface to be examined with even, coherent light and subsequently detecting the spatial intensity distribution of the radiation reflected from the surface subjected to examination.
According to one current implementation, the analysis of the diffracted light coming from the surface defect is carried out by means of a device comprising a first optical transmission system arranged to irradiate the surface to be examined with even, coherent light, and a second optical receiving system, for example a visual display or photoelectric detecting matrix symmetrically disposed relative to the first optical system with respect to the direction normal to the surface to be examined, and arranged to derive information on the spatial intensity distribution of the radiation reflected from the surface under examination.
The theoretical basis of this type of analysis is fully discussed, together with several examples of its possible uses, in chapters 4 and 7 of the work "Introduction to Fourier Optics" by Joseph W. Goodman-McGraw-Hill editions, 1968.
The analysis of diffracted light coming from surface defects, carried out by the processes described previously, is limited in use to checking flat mechanical parts and is not suitable for the qualitative checking of parts with curved surfaces, particularly of parts which have surfaces which do not possess zero curvature in at least one direction (surfaces with double curvature).
In such parts there may be observed a continuous variation, dependent on to the portion examined, in the direction of the normal to the surface, with consequent continuous variation of the direction of propagation of the radiation reflected from the surface under examination. In order to allow the analysis of the diffracted light coming from surface defects in mechanical parts of this type according to the known processess, it is thus necessary to "track" the radiation reflected from the part in space, as well as to ensure the continuously correct positioning of the analysis device at a rigidly predetermined distance from the surface to be checked.
These operating requirements cannot be achieved in practice outside the laboratory and are totally inapplicable to industrial quality control processes, particularly when this control must be effected on all the parts produced.
Moreover, whenever the parts to be inspected have a considerable size, for example motor vehicle bodywork or portions thereof subjected to painting treatments or surface protection, the need to scan the entire surface of the part makes it practically impossible to carry out a qualitative check at a frequency compatible with industrial production times.